双曲正切函数（hyperbolic tangent function）
decision tree
ID3
CART
information gain
node impurit
cross-entropy error function
quasi—Newton 拟牛顿法
BFGS(Broyden-Fletcher-Shanno-Goldfarb)
J. E. Dennis Jr. and R. B. Schnabel, Numerical Methods for Unconstrained Optimization and Nonlinear Equations. Englewood Cliffs, NJ: Prentice-Hall, 1983.
R. Setiono, “A neural network construction algorithm which maximizes the likelihood function,” Connection Sci., vol. 7, no. 2, pp. 147–166, 1995.
positive definite matrix 正定矩阵
Hessian 海森矩阵
line-search algorithm 线搜索方法